The invention relates to the method and an apparatus for automatic aspect format detection in digital video pictures.
Today, TV pictures sent by broadcasters could have different formats for the active picture (4:3, 14:9, 16:9, 2:1 . . . with or without sub-titles). When these aspect formats are transmitted in 4:3 video format, the pictures with aspect formats (14:9, 16:9, 2:1 . . . with or without sub-titles) are displayed on 4:3 TV screens with black bars. A very important function of today""s 16:9 TV""s, i.e. TV""s with 16:9 picture tube, is the possibility to zoom the received picture in order to fill the screen and remove or reduce the black bars.
In PAL+ xe2x88x92TV standard there is provided a wide screen signalling information embedded in the video signal. In most other TV standards no wide screen signalling is provided. Therefore, in most of the TV""s today, the zoom function is made manually by the user, but the user would be interested in an automatic or semiautomatic letterbox detection process which is capable of detecting the real active part of the screen in order to enable the TV to make the best zoom operation automatically. Advantageously this function has to respect the broadcasting of sub-titles with the possibility, in that case, to zoom the upper part of the screen only.
From document EP-A-0 716 542 a high precision aspect ratio auto-discrimination apparatus is known wherein it is described to detect the upper edge and lower edge of a letter box picture by detecting the average luminance of every horizontal scan period and comparing it to the average luminance mean value of a defined region in the picture. The format aspect ratio of the picture is selected in dependence of the results of the upper and lower edge detection step.
While this algorithm is very simple, there exists the disadvantage that erroneous selection operations may result in case of dark scenes with some luminous parts in the middle of the picture (e.g., tunnel scenes, . . . ). In addition, the fact to use a mean value for defined upper, lower and center regions could cover up the existence of sub-titles.
The object of the invention is to provide a robust algorithm for the automatic detection of the real active video format of the received video signal. The algorithm should provide the information which aspect format is present in the received video signal so that the zoom operation can be made accordingly.
In principle the method according to the invention has four steps: A black bars analyzing step; a center analyzing step; a horizontal transition analyzing step and a format choice step. This algorithm provides strong security when making a new decision concerning the aspect format selection and avoids unexpected or erroneous format changes. The algorithm is very simple and could be integrated in a chip at very low costs. The algorithm has very good behavior even in very noisy pictures (up to 21 dB for signal-to-noise ratio). The algorithm can react very quickly. In case of difficult pictures (dark pictures or too noisy picturexe2x80x94under 20 dB) the algorithm takes no new decision and waits for a good opportunity to make the right choice. The typical scenes which may damage the behavior of most of the known algorithmsxe2x80x94e.g., a dark scene with a luminous center part like a tunnel scenexe2x80x94do not disturb the behavior of this algorithm since the condition of recognition of horizontal transitions is not satisfied.
Advantageous developments of and improvements to the method are possible by virtue of the measures evinced herein. It is possible to detect very quickly the presence of sub-titles in the video picture (less than one second) in order to avoid consumer disturbance. A pre-processing step reduces the impact of noise in the video picture without disturbing the important information for the letter box detection. Advantageous particular embodiments are provided concerning the black bars analyzing step, center analyzing step, the sub-title analyzing step and the horizontal transition analyzing step which are easy to implement. A binary digitization step within the horizontal transition analyzing step has the advantage of avoiding adverse impact of different luminance levels on the horizontal transition analyzing step.
A gradient computation step has the advantage to allow the classification of the pixels whether they are on the correct horizontal border line, on no border line or on a wrong horizontal border line. Particular details for the gradient computation step are given. A simple method evaluates the results of the gradient computation step to find the horizontal transitions. An aspect format is selected when the horizontal transitions are found and the presence of black bars and an active picture part in the center have been detected, depending on the number of active video lines counted between the horizontal transitions. The choice can be constrained to particular format choices. A counter can be provided to distinguish for each possible aspect format so that wrong switching operations to a new aspect format are avoided when scenes feign a new aspect format. When a new aspect format has been selected, a simplified algorithm is performed wherein the ongoing presence of black bars is checked for the selected format.